Clouds are the Clue for Better Climate Predictions
PITTSBURGH, May 25, 2016 /PRNewswire/ -- Climate predictions are based on three components: greenhouse gas emissions, clouds and atmospheric aerosol particles, and solar variability. Scientists can accurately determine greenhouse gas emissions and variations of the sun, including sunspot cycles and variations in cosmic rays. However, clouds and aerosols are an area about which scientists are uncertain, because they do not fully know how aerosol particles form and how they affect clouds.
Two new papers published in Nature on May 26 from a team at the CERN CLOUD experiment led by researchers from the Paul Scherrer Institute and Carnegie Mellon University's College of Engineering and Mellon College of Science demonstrate how aerosol particles form, allowing for much more accurate climate predictions.
Clouds and aerosol particles are essential to climate predictions because they reflect sunlight back into space, which cools the planet—unlike the other aspects of climate change, which warm the planet. Therefore, clouds mask some of the warming caused by humans. The problem is that scientists do not know how much clouds mask warming because, due to not fully understanding how clouds form, they have been unable to predict how clouds and aerosols have changed from pre-industrial times to now.
Clouds develop when water vapor in the atmosphere condenses on particles—the water vapor needs something to attach to. These tiny particles are called cloud condensation nuclei (CCN). These particles have major health effects and are responsible for three of the top ten sources of death globally.
To learn more about CCNs, visit: https://youtu.be/cn_gjU7AiMs.
"We are not seeing some of the warming in place because it is masked by this haze that is killing some seven million people per year," says Neil Donahue, professor of chemical engineering, engineering and public policy, and chemistry at Carnegie Mellon University. "The best estimate is that about one-third of the warming by greenhouse gas emissions is masked by this aerosol cooling, but the fraction could be as large as half and as little as almost nothing."
Some particles are emitted directly—for example, as smoke from fires or sea spray—while others form literally out of thin air in a process called nucleation. All must grow to about 100 nm in size to become CCN. Until now, scientists did not completely understand nucleation in the atmosphere—but the two papers provide models that demonstrate how nucleation occurs and how all particles grow.
Freshly nucleated particles contain only a few molecules, and these particles grow very slowly. Tiny particles also move around more than big particles, and most of the small particles bump into larger particles and die. They only survive if they grow fast enough.
So how do the particles grow? One paper, "Low-volatility organic compounds are key to initial particle growth in the atmosphere," deals directly with this question.
Some organic compounds are very sticky (less volatile), meaning they adhere easily even to the smallest particles, while other compounds are less sticky. The less sticky ones (more volatile) do not adhere to the tiny particles, but they will adhere as the particles get larger.
The group found that when a compound emitted by trees is oxidized in the atmosphere, it makes only a few extremely sticky compounds and more that are somewhat less sticky, resulting in particles that grow slowly at first. This means for the first few hours, the particles are particularly vulnerable to dying by bumping into large particles. As the particles grow, the growth accelerates, and the particles can grow to become CCN. The CMU researchers were able to reproduce the growth rates observed in the CLOUD experiment with a model that describes the wide range of stickiness (volatility) of molecules produced by chemistry in the atmosphere.
The second paper, "Ion-induced nucleation of pure biogenic particles," explores the popular scientific theory that particles require sulfuric acid vapor, which is largely associated with human activity, to form. The group found that this is not true.
"We found new particles can form exclusively from molecules derived by the oxidation of the compound alpha-pinene, which is emitted by vegetation and responsible for the characteristic smell of pine trees," says Donahue. Another way new particles can form is through extremely sticky particles, especially with the help of ions formed by cosmic rays.
"This refutes the idea that there may be more particles in the atmosphere today due to pollution than there were in 1750 and suggests that baseline pristine pre-industrial climate may have had whiter clouds than presently thought," says Donahue.
The team's research has lasting climate implications, especially considering the international COP21 agreement that commits all countries to a target of no more than two degrees Celsius global warming from the greenhouse effect.
"Earth is already more than 0.8 degrees Celsius warmer than it was in the pre-industrial epoch, and this is with some masking by aerosol particles. As the pollution subsides, up to another 0.8 degrees Celsius of hidden warming could emerge," says Donahue.
About the College of Engineering: The College of Engineering at Carnegie Mellon University is a top-ranked engineering college that is known for our intentional focus on cross-disciplinary collaboration in research. The College is well-known for working on problems of both scientific and practical importance. Our "maker" culture is ingrained in all that we do, leading to novel approaches and transformative results. Our acclaimed faculty have a focus on innovation management and engineering to yield transformative results that will drive the intellectual and economic vitality of our community, nation and world.
About Carnegie Mellon University: Carnegie Mellon (www.cmu.edu) is a private, internationally ranked university with programs in areas ranging from science, technology and business to public policy, the humanities and the arts. More than 13,000 students in the university's seven schools and colleges benefit from a small faculty-to-student ratio and an education characterized by its focus on creating and implementing solutions for real world problems, interdisciplinary collaboration and innovation.
SOURCE CMU College Of Engineering
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